Add a shadowing guide for SIG Contribex blogging

[fsmunoz]

Describe the issue

Since we're taking shadows, it would make sense to have a document that would serve as a guide, identifying what is expected, in a similar way to what exists for e.g. Kubernetes release shadows.

Content could include:

• Typical tasks
• Useful skills
• Time expectations

[fsmunoz]

/assign

[fsmunoz]

Initial draft: https://github.com/fsmunoz/community/blob/contribex-comms-blog-shadow-guide/communication/contributor-comms/blogging-resources/blogging-shadow-guide.md

[fsmunoz]

/area contributor-comms /sig contributor-experience